1. Field of the Invention
The present invention relates to a novel organic electrochromic compound and an electrochromic element including the same.
2. Description of the Related Art
Various materials have been reported as electrochromic (hereinafter, may be abbreviated to “EC”) materials that vary their optical absorption properties (colored state and light transmittance) by electrochemical redox reactions. Though metal oxides such as WO3 are known as inorganic EC materials, the method of forming a film from such a metal oxide is limited to, for example, deposition, which disadvantageously prevents formation of a large-size film.
As organic EC materials, for example, conductive polymers such as polythiophene and polyaniline and low-molecular-weight organic compounds such as viologen and oligothiophene are known.
Examples of the low-molecular-weight organic EC compounds include viologen derivatives (cathodic compounds) coloring by reduction and oligothiophene derivatives (anodic compounds) coloring by oxidations.
These low-molecular-weight EC compounds have a shorter π conjugation length compared to the conductive polymers and have absorption in the ultraviolet region. The compounds therefore transmit visible light in a neutralized state and absorb visible light in an oxidized state (in a case of anodic compounds) or in a reduced state (in a case of cathodic compounds). That is, since the conjugation length in an oxidized state or a reduced state is longer than that in a neutralized state, the wavelength region absorbing light shifts to the visible light region.
In other words, the low-molecular-weight EC compounds discolor in a neutralized state and color in an oxidized state or a reduced state.
Japanese Patent Laid-Open No. 9-120088 describes a viologen derivative coloring in a reduced state. An oligothiophene derivative that colors in an oxidized state is described in GUAY, J; KASAI, P; DIAZ, A, et al., Chemistry of Materials, 1992, vol. 4, No. 5, pp. 1097-1105. U.S. Patent Application Publication No. 2013/0100517 describes an organic compound having dithienothiophene and describes an electrochromic element including the organic compound having dithienothiophene.
An EC element including a low-molecular-weight organic EC compound shows high transparency in its discoloring. As an EC element including a low-molecular-weight organic EC compound, a sandwich-type element in which an EC compound dissolved in a solvent such as propylene carbonate is disposed between a pair of electrodes is known.
The optical density (low transmittance) in the coloring of a low-molecular-weight EC compound can be increased by dissolving the low-molecular-weight EC compound in a solvent at a high concentration.
The oligothiophene derivative described in Chemistry of Materials, 1992, vol. 4, No. 5, pp. 1097-1105 has a low solubility in a solvent and is unstable when a redox reaction is repeated.
The viologen derivative (cathodic compound) described in Japanese Patent Laid-Open No. 9-120088 has a high solubility due to its molecular structure, but is unstable when a redox reaction is repeated. The organic compound described in U.S. Patent Application Publication No. 2013/0100517 has insufficient solubility in a solvent such as propylene carbonate.